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Pneumonia seems to be a constant stalker of calves, and for good reason. Most of the bacterial pathogens that cause it are present in the calf’s upper respiratory tract; they just need the opportunity of an immune system compromised by viral infections or stress.

Summer pneumonia is among the most frustrating respiratory infections in nursing calves. The cause is elusive, and many cases occur during seemingly non-stressful conditions for the calves.

A multistate team of researchers is studying ranches in Nebraska and the Dakotas affected by the problem, and comparing them with ranches in the same veterinary practices that haven’t experienced the disease. The team includes Russ Daly, South Dakota State University; Jerry Stokka, North Dakota State University; David Smith, currently at Mississippi State University but formerly at the University of Nebraska; and Amelia Woolums, University of Georgia (UGA).

Smith, an epidemiologist, investigated outbreaks in Nebraska herds for several years. He consulted with Woolums to design the study because of her expertise in immunology and vaccination to control respiratory disease in cattle.

“We know a lot more about weaning and post-weaning respiratory cattle disease in calves, and how to prevent and treat it,” Daly says. “Basically, the lesson is that if we minimize calves’ stress levels and do a good job of early vaccination, we tend to see fewer problems,” he says.

However, there’s little data regarding the risk factors for summer pneumonia. “We want to understand why some ranches perennially have a problem, and others never do,” Woolums says.

“There may be management practices in some herds that put cattle more at risk. For instance, gathering cattle for artificial insemination (AI), sorting, exposure to dust, etc., may favor virus transmission or compromise the immune system. We don’t always know which factors might be important,” Smith says. In some herds, the pneumonia occurs in calves out on the range, absent of those activities.

“We’re looking at farms with summer pneumonia cases and comparing them with farms in a similar location and time that haven’t had issues,” Daly says. “We want to discover the factors that favor calf health on some farms, or that could help producers prevent this disease. Of course, environmental factors are hard to control, but at this point, we don’t even know if these are involved.”

He says some operations have problems in June and July. Others have sickness in very young calves, while some have pneumonia in calves a month before weaning. While these latter calves will respond to treatment, their weight gain takes a hit.

“If pneumonia happens close to weaning, that calf might not be able to be sold with the rest of the group. There’s also the issue of long-term damage and scarring in the lungs; those calves just don’t do as well later,” Daly says.

Looking at multiple factors

The study is looking at various factors, such as gathering cows and calves for AI or branding. “The calves are only apart from their mothers for a short time. We wouldn’t think it could be enough stress to affect them, but perhaps this handling facilitates transmission of bacteria or viruses,” Daly says.

Or perhaps multiple factors are at work. On some operations, for instance, ranchers move cattle for intensive grazing, or subpopulations are commingled at different times.

“Commingling cattle from different sources is a well-known risk factor for feedlot bovine respiratory disease. We don’t know if commingling young calves from different subpopulations on the same ranch increases the risk,” Woolums says.

When cattle on the range are moved to new pastures, stress could be a factor if it’s hot, dusty or a long drive. But what about cattle that aren’t being moved or handled?

“Cattle diseases like calf scours are often multi-factorial; summer pneumonia may be, too,” Daly says. Colostrum intake has been looked at, along with calves’ ability to respond to vaccine at different ages. Weather, like a late-spring storm or a cold, rainy spell in early summer, might be a contributing factor in some situations.

“A respiratory pathogen may be circulating, with some unapparent mild infections, and a little extra stress might put them over the edge,” Smith says.

There may be genetic differences in calves’ ability to mount a good immune response as well. “We’re just scratching the surface of that possibility,” Daly says. “We know crossbred cattle have hybrid vigor, and part of that characteristic includes hardiness and disease resistance.”